There is a centrifugal compressor in which an end portion of a rotary shaft protrudes to the outside of a casing so as to input or output a rotation of the rotary shaft that is rotatably provided with respect to a casing. In the centrifugal compressor, a dry gas seal is provided so as to inhibit a working fluid from leaking a gap between the rotary shaft and the casing in the end portion of the casing to the outside.
For example, a compressor having the dry gas seal is disclosed in Patent Document 1. The dry gas seal disclosed in Patent Document 1 includes a drive system unit that integrally rotates with a rotary shaft and a stationary system unit that is fixed to a head of a casing. In the dry gas seal, a stationary ring of a stationary system unit is biased toward a rotating ring of the drive system unit by a coil spring. Accordingly, in a state where the compressor is stopped, the stationary ring and the rotating ring abut each other. In addition, a spiral groove is formed on a seal surface of the rotating ring facing the stationary ring. If the rotary shaft rotates in a state where the compressor is operated and a seal gas is supplied, the seal gas is introduced to a portion between the rotating ring and the stationary ring by the spiral groove. The stationary ring is pressed along an axial direction of the rotary shaft against a biasing force of the coil spring by a pressure of the seal gas. As a result, a minute gap is formed between the rotating ring and the stationary ring, and thus, the portion between the rotary shaft and the casing is sealed without inhibiting the rotation of the rotary shaft.
Meanwhile, the seal gas supplied to the dry gas seal flows through the minute gap between the rotating ring and the stationary ring. Accordingly, if a portion of the seal gas is liquefied, the minute gap is clogged, and thus, performance of the dry gas seal greatly deteriorates. Therefore, in order to keep the seal gas at a temperature equal to or more than a dew point, it is necessary to install gas heater for heating the seal gas.